The present invention relates to a method of manufacturing a semiconductor device and for example, to a method of manufacturing a semiconductor device having a nonvolatile memory.
EEPROMs (Electrically Erasable and Programmable Read Only memories) are widely used as nonvolatile semiconductor storage devices which electrically enable writing and erasing. Such storage devices, typified by flash memories which are widely used at present, have a conductive floating gate electrode surrounded by an oxide film or trap insulating film under the gate electrode of a MISFET, in which the state of charge storage in the floating gate or trap insulating film is taken as storage information and read as a transistor threshold. The trap insulating film refers to an insulating film which can store a charge and one such example is a silicon nitride film. The threshold of the MISFET is shifted by charge injection into, or charge emission from, such charge storage region so that the MISFET operates as a storage element. Among the nonvolatile semiconductor storage devices which use a trap insulating film are split-gate cells which use a MONOS (Metal Oxide Nitride Oxide Semiconductor) film.
In the category of MISFETs (Metal Insulator Semiconductor Field Effect Transistors), as a technique to prevent generation of hot carriers due to a high electric field, it is known that the source and drain regions have an LDD structure which includes a semiconductor region with a low impurity concentration and a semiconductor region with a high impurity concentration.
Japanese Unexamined Patent Application Publication No. 2008-153567 describes that a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is formed over a SOI (Silicon On Insulator) substrate. In this technique, the source diffusion layer of the MOSFET has a higher impurity concentration than its drain diffusion layer.
Japanese Unexamined Patent Application Publication No. 1999-144483 describes that in a memory cell as a MOS (Metal Oxide Semiconductor) transistor, the drain region extends more widely than the source region.